The present invention relates generally to devices for detecting very small magnetic fields and magnetic field gradients and more particularly, to devices for detecting such small magnetic fields and gradients by means of optical fiber light modulation.
Magnetic field sensors may be utilized in a variety of applications such as sensing the earth's magnetic field, warfare sensing applications, and in Geophysical earth exploration. Of particular interest, is the applicability of these devices for use in anti-submarine or anti-ship warfare and mine detection. Typically, the magnetic fields set up by the metallic structure of moving ships or submarines is quite small, on the order of 10.sup.-7 gauss or less.
Presently, the best performance for detection of small magnetic fields in the 10.sup.-7 -10.sup.-10 Gauss range is obtained with superconducting quantum interference (SQUID) devices. However, since SQUID magnetometers are cryogenic devices which must be cooled to 4.degree. K., they require considerable support and maintenance in comparison with conventional electronic devices which operate at ambient temperatures. In addition. SQUID magnetometers are expensive (up to $50,000 per instrument), have low reliability, and their performance degrades severely in high-vibration military environments. Recently the use of a fiber optic interferometer as a sensitive magnetometer was proposed (Yariv and Winsor, Opt. Lett. 5, 87 (1980)) and demonstrated (Dandridge, et al., Electron. Lett., 16, (1980)). Those references describe a device in which a magnetostrictive material introduces a strain proportional to the ambient magnetic field into a sensing fiber. This strain changes the optical path length of one arm of the interferometer relative to that of the other arm, leading to a change in relative intensities of the optical outputs from the interferometers. The interferometer output beams are sensed by photodetectors, which provide an electrical signal from which the amplitude of the ambient magnetic field can be determined.
Unfortunately, such interferometers are also very good sensors of temperature and thermal gradients, acoustic pressure, vibration and other environmental affects. Thus, in order to reach the theoretical sensitivity of 10.sup.-12 Gauss expected with no environmental affects present, it is necessary to eliminate or cancel out these environmental effects. However such cancelation is difficult to achieve in a practical instrument.